Recently when I drive I noticed this noise which tends to come around 2000rpm. Going through the gears I can't hear anything through first and second, then it starts in this rev band through 3rd and especially 4th at lower speeds say 35mph-45mph. I looked under the car at the muffler to see if there was a hole under it but it looks sound. Any idea what might be causing the problem? Its not got any worse. I drive about 40 miles in it per day. It is a manual.